1. Field of the Invention
This invention relates to a glazing unit alarm system of the kind comprising electric alarm means and a glazing unit incorporating at least one glazing panel having spaced apart faces, the alarm means including electrically conductive means applied to said at least one glazing panel and comprising a light transmitting electrically conductive coating extending across a first face of said at least one glazing panel and detecting means for detecting a change in resistance of the electrically conductive means occasioned by a breakage or an attempted breakage of the glazing unit. The invention is primarily, although not exclusively, intended for application in double glazing units.
2. Description of Related Art
A known glazing unit alarm system of the kind referred to is disclosed in GB-A-2,169,426. In this known alarm system the electrically conductive coating is in the form of an emissivity or solar control coating and serves the dual function of acting as a heat radiation filter with a cut off in the infra red region of the electromagnetic spectrum and as a part of the electric circuit of the electric alarm means. The cut off is suitably operative at a wavelength of from 2 .mu.m to 10 .mu.m, e.g. about 3 .mu.m, and the coating transmits the majority of heat radiation incident thereon which has a wavelength on one side of the cut off wavelength and reflects the majority of heat radiation incident thereon which has a wavelength on the other side of the cut off wavelength. Normally the coating acts to transmit the majority of solar heat radiation and reflects the majority of the longer wavelength heat radiation produced in buildings by lights, radiations, absorbed and re-radiated solar energy and body heat.